That's until the appeals court or Michigan Supremes(strict constitutionalist) slaps them down .
It looks like the court ruled that another "law of this state" -- one authorizing cities to protect their buildings -- 'provides otherwise' and allows cities to prohibit firearms in their buildings: "But according to the state law cited by Karie Boylan, a Livonia lawyer retained by the city, local municipalities "can enact and enforce ordinances and resolutions for the care, protection, control and management" of city-owned buildings."
If that's what happened, then I don't think winning on appeal is such a slam dunk. Of course, that begs the question of whether it's Constitutional or not; but it'll be the preemption statute, not the 2nd Amendment, that'll be argued on appeal, and there's a pretty good argument that the Ferndale ordinance passes muster under the preemption statute because of the "except as otherwise provided by state law" language.
OTOH, I've been wrong before.